1. Field of the Invention
The present invention relates to a flux-cored wire for gas-shielded arc welding of a galvanized steel sheet, and in particular to an improved pit and blow hole resistant flux-cored wire for gas-shielded arc welding of a galvanized steel sheet which can efficiently prevent pits and blow holes from being generated in the gas-shielded arc welding of the galvanized steel sheet, by filling a flux containing titanium oxide, silicon oxide and alkali fluoride as slag generation agents in a mild steel sheath in an amount of 10 to 20% by weight of the wire.
2. Description of the Related Art
In general, the most widely-used welding method is an arc welding which combines base materials by generating an arc heat by applying a low voltage high current power between a welding material(electrode or filler metal) and a non-welding material(base material), and by melting the two materials by the arc heat. The arc welding is classified into the following two types.
In a consumable electrode arc welding, as illustrated in FIG. 1, a welding rod 11 served as an electrode and a base material are melted due to an arc heat 12 generated between the electrode and the base material, thus forming a weld metal 13, namely a bead on a weld zone. The consumable electrode arc welding includes a shielded arc welding, submerged arc welding, carbon dioxide arc welding and MIG welding.
In a non-consumable electrode arc welding, as shown in FIG. 2, an electrode 21 serves merely to generate an arc 22. It is thus required to melt a specific filler metal 23 in an arc generation range in order to obtain a weld metal. The non-consumable electrode arc welding includes a TIG welding and atomic hydrogen welding.
The arc welding methods except for the shielded metal arc welding (SMAW) prevent oxidation of a welding section where a liquid phase metal is formed due to the arc heat, or shield a contact between a molten metal and air by supplying an inert gas or carbon dioxide gas to the welding section so as to control an atmosphere of the molten metal. Accordingly, the arc welding methods are also called the gas-shielded arc welding.
The gas-shielded arc welding of high efficiency has been widely employed in various industrial fields such as vehicles, shipbuilding and construction. However, the welding of iron and steel materials has the following problems.
The iron and steel materials have excellent mechanical properties in spite of low price, but have a weak corrosion resistance. Therefore, in order to improve the corrosion resistance, the surface of the steel material may be galvanized with pure zinc or zinc alloy.
However, the plated layer on the surface of the iron or steel material may generate defects in the weld zone.
That is, the plated layer of the galvanized steel material consists of zinc having a low melting point and boiling point. As compared with the pure steel material, the arc weldability of the galvanized steel material is remarkably reduced, and welding defects are easily generated in the weld zone due to the following reasons.
The zinc of the welding section and the plated layer of a heat effect unit is heated over its boiling point by the heat generated in the arc welding. Here, the evaporated zinc gas penetrates into the molten metal of the welding section.
Accordingly, the zinc gas penetrating into the molten metal of the welding section remains in the bead after coagulation of the molten metal, thereby forming a space of an internal residual gas, namely a blow hole. In addition, the residual gas is discharged through the surface of the bead which is not completely coagulated, thereby forming a pit thereon.
The blow hole or pit generated due to the zinc gas weaken the weld zone, and thus reduce intensity of the steel material or cause cracks.
A conventional flux-cored wire for the gas-shielded arc welding of the galvanized steel material has been mostly used for the galvanized steel material for vehicles or light-weighted steel frames whose plated layer has a thickness below 20 xcexcm.
However, when the flux-cored wire is used for the welding of a plated steel material whose plated layer has a thickness of 50 to 500 xcexcm, a steel pipe whose plated layer has a groove, or a galvanized steel pipe such as an air conditioning pipe of ships, the blow holes or pits are seriously generated. In addition, the molten metal of the weld zone closes a supply nozzle of a shielding gas due to serious spattering, thereby reducing gas shield efficiency. It is thus difficult to consecutively perform the welding process.
Therefore, in order to stabilize the welding, the plated layer of the weld zone must be removed by grinding before the welding, the vaporized zinc gas must be easily discharged by increasing root intervals of the I-type joint welding, or zinc of the plated layer must be vaporized in advance due to increased preheating of a first welding, and then processed in a second welding or so. As a result, the weldability is considerably reduced.
Accordingly, an object of the present invention is to provide a flux-cored wire containing optimized flux components for welding which can restrict generation of pits and blow holes in the gas-shielded arc welding of a galvanized steel material, and which can control fumes and spattering to consecutively perform a welding process.
In order to achieve the above-described object of the present invention, there is provided a flux-cored wire for gas-shielded arc welding of a galvanized steel sheet wherein an amount of flux ranges from 10 to 20% by weight of the wire, an amount of a mild steel sheath ranges from 80 to 90% by weight of the wire, and a total amount of iron powder, deoxidizer and arc stabilizer in the flux ranges from 5 to 15% by weight of the wire, the flux comprising such components being filled in the mild steel sheath.